Proximity effect and interface transparency in Al/InAs-nanowire/Al diffusive junctions

Abstract

We investigate the proximity effect in InAs nanowire (NW) junctions with superconducting contacts made of Al. The carrier density in InAs is tuned by means of the back gate voltage Vg. At high positive Vg the devices feature transport signatures characteristic of diffusive junctions with highly transparent interfaces -sizable excess current, re-entrant resistance effect and proximity gap values (∆N ) close to the Al gap (∆0). At decreasing Vg, we observe a reduction of the proximity gap down to ∆N ≈ ∆0/2 at NW conductances ∼ 2e 2 /h, which is interpreted in terms of carrier density dependent reduction of the Al/InAs interface transparency. We demonstrate that the experimental behavior of ∆N is closely reproduced by a model with shallow potential barrier at the Al/InAs interface.A possibility to interface a superconductor (S) with a metallic (N) state of InAs-based mesoscopic semiconductors is known since at least two decades 1,2 . High critical and excess currents, sometimes comparable to a theoretical upper limit, are reliably observed in InAs nanowire (NW) Josephson junctions 3-5 . Observations of signatures of a Cooper pair splitting 6,7 further conform with high device quality. A recent revival of interest in proximity effect in S-NW devices emerged in the course of a search for Majorana zero-energy states 8 , with the research targeted at single conduction channel NWs 9-12 .Thanks to a well-known Fermi level pinning effect, which results in a charge accumulation at the surface of InAs 13,14 , the NWs grown from this material typically exhibit low contact resistance to normal metals. For the superconducting hybrid devices, however, even a weak interface reflectivity can sufficiently degrade the performance by suppressing the Andreev reflection in favor of the normal quasiparticle scattering 15,16 . Although the transparency of the S/NW interfaces can be controlled by various chemical treatments prior to the deposition of the superconductor 3-5,17 or, alternatively, via the in-situ MBE growth of the superconductor 18 , residual imperfections are still present 19,20 . One might expect that the underlying physics can be clarified by tracking the carrier density dependence of the proximity effect, since the effective interface transparency should critically depend on the shape of the potential barrier.In this work we study the evolution of the superconducting proximity effect in an InAs NW with evaporated Al contacts as a function of electron density in InAs, which is controlled by means of a global back gate. At high carrier densities, the transport data is compatible with highly transparent Al/InAs interfaces and the induced proximity gap (∆ N ) is close to the superconducting gap value in Al (∆ 0 ). The observed reduction of ∆ N by a factor of two at decreasing carrier density indicates gate-tunable interface transparency, which can be explained assuming a shallow potential barrier formed at the Al/InAs interface.We investigate unintentionally doped catalyst free InAs NWs grown by MBE on a Si(111) substrate. ...

InAs/MoRe Hybrid Semiconductor/Superconductor Nanowire Devices

“…We note that the magnitude of the induced gap Δ* = 0.9 meV is lower than the value Δ bulk = 1.75 k B T C = 1.4 meV expected from the transition temperature T C = 9.9 K of the MoRe film (Figure a). This is similar to MoRe/graphene devices showing a reduced Δ* = 1.2 meV. , In analogy with results from Nb-based devices, this could be a consequence of oxidized or contaminated layers at the nonideal InAs/MoRe interface, ,,, defects in the superconductor, and/or carrier-density-dependent interface transparency . Alternatively, the larger Δ bulk leads to a shorter coherence length − L C B = ℏ v F /Δ bulk for ballistic transport and L normalC normalD = ℏ v normalF l normale / 2 Δ bulk for diffusive transportsetting the scale beyond which Δ* decays from the contact. − Taking a typical Fermi velocity v F ≈ 2 × 10 5 m/s and mean free paths l e = 50–200 nm for InAs and Δ bulk = 1.4 meV, we find L C B = 100 nm and L C D = 50–100 nm, smaller than the electrode separation (130 nm for device 1).…”

“…18,20 In analogy with results from Nb-based devices, this could be a consequence of oxidized or contaminated layers at the nonideal InAs/MoRe interface, 9,12,24,30 defects in the superconductor, 34 and/or carrier-density-dependent interface transparency. 35 Alternatively, the larger Δ bulk leads to a shorter coherence length…”

“…18,20 In analogy with results from Nb-based devices this could be a consequence of oxidized or contaminated layers at the non-ideal InAs/MoRe interface 9,12,24,30 , defects in the superconductor 34 , and/or carrier density-dependent interface transparency. 35 Alternatively, the larger ∆ bulk leads to a shorter coherence length -L B C = v F /∆ bulk for ballistic transport and L D C = v F l e /(2∆ bulk ) for diffusive transport -setting the scale beyond which ∆ * decays from the contact. [36][37][38] Taking a typical Fermi velocity v F ∼ 2 × 10 5 m/s and mean free paths l e = 50 − 200 nm for InAs and ∆ bulk = 1.4 meV, we find L B C = 100 nm and L D C = 50 − 100 nm, smaller than the electrode separation (130 nm for Dev.…”

Noise insights into electronic transport

“…Indeed, in NW1 in B = 0 T we observe the increase of G at sub-gap voltages, starting abruptly at approximately twice the superconducting energy gap of Al: |V | ≈ 2∆/e ≈ 380 µV, see vertical arrows. Microscopically, this sub-gap enhancement above the normalstate conductance G N reflects the Andreev reflection charge transport taking place at Al/InAs interfaces and is distinctive for transmissive SNS structures 76,77 . The observation of the supercurrent/large zero-bias conductance G S = G(V = 0, B = 0) further suggests the good quality of the interfaces in the device NW1.…”

Special Issue on hybrid quantum materials and devices

“…Hybrid superconductor/semiconductor/topological insulator systems Proximity-effect hybrid devices provide the perfect example of how the combination of different quantum materials can be used to create devices with qualitatively new properties. The paper by A V Bubis et al addresses the issue of contact transparency in nanowire Josephson devices, which has an important impact on how effectively pairing correlations can be transferred between two materials [5]. Clearly, important perspectives come from the combination of pairing with more exotic quantum materials.…”